Molded pulp product and process



United States Patent 3,085,040 MOLDED PULP PRODUCT AND PROCESS David W.Levering, Necdham, and Alton B. Poole, Abington, Maine, assignors toArthur D. Little, lire, Cambridge, Mass, a corporation of MassachusettsNo Drawing. Filed Feb. 6, 1961, Ser. No. 37,994 5 Claims. (Cl. 162-472)This invention relates to molded pulp articles. More particularly itrelates to molded pulp article of high resistance to the penetration ofoils, fats and greases over a wide temperature range and to the methodof producing the same.

Articles molded of cellulosic fibers, and more particularly frompapermaking fibers, can be made into attractive and inexpensive dishesand food containers. Moreover, it is most desirable that such articlesbe able to hold both hot and cold food without permitting penetration ofthe liquid content, both water and oils, of the foodstuff. However, theprior art has left much to be desired insofar as articles of this typewhich have good liquid penetration resistance, particularly resistanceto the penetration of oils and fats over the entire temperature range atwhich most foods are served or stored.

Attempts have been made in the prior art to render molded pulp articlesresistant to the penetration of oils and fats at various temperaturesbut these have only met with limited success. For example, molded pulparticles treated with a low melting point wax or paraifin provideresistance to the penetration of oils or fats at room temperature orslightly above, but oils at temperatures of about 185 F or greaterpenetrate these articles almost immediately, making them unsatisfactoryfor holding hot foods or other hot oily substances. On the other hand,molded pulp articles treated with a high melting wax (MP. 195 F.) can bemade resistant to the penetration of hot oils or fats at temperaturesbelow l95 F., but these are found unsatisfactory for holding materialscontaining oils or fats at temperatures less than about 150 F. Variousattempts to treat pulp articles with a mixture of low and high meltingpoint waxes have not been successful because in such processes thesewaxes have been blended and merely result in a mixture having a meltingpoint lying between the respective melting points of the two waxesmaking up the mixture.

The term oil will hereinafter be used to include oils of synthetic,mineral, vegetable and animal origin, greases, fats and the like,whether they are encountered in cooked foods or other substances.

It is an object of this invention to provide a molded pulp article whichis resistant to the penetration of oils applied thereto over a Widetemperature range. It is another object to provide a molded article ofthe character described which is also resistant to the penetration ofother liquids, e.g., water. It is yet another object of this inventionto provide an improved container of molded pulp which is capable ofholding both hot and cold foods and preventing penetration of the oil orother liquid content thereof during its use. Still another object is toprovide molded articles of the character described in which thepenetration of oils above 195 F. can be controlled or prevented over anextended period of time. Another and important object of this inventionis to provide a method of producing a molded cellulosic article which isresistant to the penetration of hot and cold oils. These and otherobjects will become more apparent upon considering the followingdescription of the present invention.

The product of this invention may be described as a molded pulp producthaving substantially uniformly distributed therethrough from about 1% toabout 16% by Patented Apr. 9, 1963 ice weight of a mixture of a firstwax melting above about 212 F. and a second wax melting below about 165F., the amount of said first wax in said mixture being in excess of thatamount which is compatible with said second wax when said waxes aremelted together and subsequently solidified.

As will be explained more fully hereinafter, the wax components areintroduced into the molded article by incorporating the same into thefiber or pulp stock prior to the actual molding of the article. This ismost conveniently accomplished by the use of emulsions or dis persionsof the waxes.

The commonly used hydrocarbon waxes are known to be made up of a mixtureof a large number of different hydrocarbon molecules varying in thenumber of carbon atoms contained. This in turn means that the varioushydrocarbon waxes are compatible, and blend may be made to obtain ahydrocarbon wax having a desired melting point. In true wax blendsessentially all of the blend melts at the same temperature, thattemperature being intermediate between the melting points of the waxesgoing to make up the blend and determined by the percent and meltingpoint of each of the waxes used in the blend. The use of two or more ofsuch compatible waxes which behave in this manner explains the inabilityof prior art processes to achieve the wide range of protection againstoil penetration in molded pulp articles now achieved by the practice ofthis invention.

In contrast to the prior art, this invention requires the use of twowaxes which have widely spread melting points and which exhibit a degreeof incompatibility toward each other. This required degree ofincompatibility is used to provide in the finished molded pulp articleprotection against oil penetration over a Wide range defined asextending from essentially the lower temperature limit attainable by thelow melting point wax when used alone up to the higher limit attainableby the high melting point wax when used alone. This desired protectionis in turn due to the presence in the molded pulp article of waxeshaving a spread of melting points, there being present distinct phasesof waxes substantially identical to the low melting wax, and the highmelting wax and a spectrum of wax phases having melting pointsintermediate of the melting points of the two wax components. Thus,there is achieved not only a wide range of temperature protection butalso complete protection within this range.

The compatibility or incompatibility of waxes may be recognizedoptically or by plotting a solidification curve for a mixture of two ormore waxes as they 0001 from a molten condition. if, for example, twowaxes are incompatible, a plot of temperature against time will show twobreaks in the curve, one of which occurs at approximately the meltingpoint of the higher melting point wax, the other at the melting point ofthe lower melting point wax. A limited degree of compatibility of thetwo waxes is evident by an intermediate break or breaks in the curve.These intermediate breaks in the cooling curve indicate that the heat ofcrystallization of a compatible compound is retarding the cooling rate.For example, in the preterrcd waxes used in the practice of thisinvention, it has been found that the greatest degree of compatibilityis achieved when approximately by weight of total wax is of the lowmelting component and 15% by weight of the high melting component. Theperformance of our finished molded pulp articles, in terms of ability tofurnish protection from oil penetration throughout the entiretemperature range, demonstrates that there are a multiplicity of suchcompatible mixtures giving a multi plicity of waxes, each having aslightly different melting point. In addition, however, the finishedpulp article also contains portions of the high melting point wax and ofa) the low melting point wax in to their original form.

The high melting point wax used in this invention is defined as onehaving a melting point above 2l2 F. Since temperatures much in excess of308 F. would begin to degrade the molded pulp article itself, it is unnecessary to employ a high melting point wax having a melting pointgreater than 300 I. his high melting wax component may be furtherdefined one which exhibits the required degree of incompatibility to thelow melting wax.

Among the high melting point waxes which are suitable for the practiceof this invention, the preferred wax is Acrnwax C (sold by GlycoChemicals). Acrawax C is a complex nitrogen derivative of the higherfatty ac and has a melting point of about 284 F. It is undcr stood to beessentia ly ethylene bi teammide). Likewise, methylene biststcarnmide)and ethylene bis(stearam-- ide) may be used as well as any otherwax-like material conforming to the requirements given above. Amongother suitable commercially available high melting NN ethylenebis(steuramidc) waxes may be listed Alrowax 140 which has a meltingpoint of 284 F. (sold by (ieigy Chemical Corporation) and Carlisle wax230 which has a melting point of 280-29tl F. (sold by Carlisle ChemicalWorks, Inc.).

The low melting point wax component is one which has a melting point of165 F. or less but preferably not below about 50 F. The lower limitplaced on the melting point of this lower melting wax component isdetermined by the temperature of the oil against which this waxcomponent is to protect the molded ccllulosic article. Under normalcircumstances, for example, where the cellulosic article is a foodcontainer, the lower temperature oils or fats will be at approximatelyroom temperature. It would not, of course. be normally desirable to havea wax present in the molded cellulosic article which softens or melts atroom temperature or below. This. therefore, indicates a practical lowerlimit of about 5t) F. for the low melting point wax component. A varietyof known low melting point waxes are suitable for the practice of thisinvention. Hydrocarbon waxes, the microcrystalline waxes, beeswax, andthe like are suitable low melting point waxes.

If the cellulosic article, treated in accordance with this invention, isto be exposed over prolonged periods to the atmosphere, or oxidutiveconditions, it is preferable that the low melting point wax componentcontain a minimum amount of liquid hydrocarbons, i.e., not more thanabout 15 by weight, Waxes which have liquid hydrocarbons below thislevel are obtainable, or they my be treated to remove these liquidcomponents by means well known to those skilled in the art.

The total amount of wax used should be that which will provide at leastabout 1% by total weight in the molded pulp article. Although a smalleramount of wax may be used with acceptable results, it is difficult tophysically distribute less than about 1% uniformly throughout thefibers. It is not considered economically practical to use more thanabout 16% by weight. The amount of high melting point wax in the waxmixture must be in excess of that amount which is compatible with thelow melting point wax when the waxes are melted together andsubsequently solidified.

Th pulp used to form the molded article is prepared in accordance withknown techniques which include formation of a water-fiber slurry. 'l'hewaxes used in this invention may be added at any suitable point duringthe slurry formation, preferably to the beater stock.

Because of the very high melting point of the high melting waxcomponent, it is often ditficult to form a suitable water emulsion forintroducing it into the fiber stock. Therefore, the finely-divided highmelting point wax component is most conveniently formed into a waterdispersion and added in this form to the slurry. Addition of formscsscnti. ily identical the dispersion is preferably made to the stock inthe beater. Of course, in those cases where an emulsion can be made withthe high melting point wax component, it

is be used in place of the dispersion.

In making a dispersion of the high melting wax component to be added tothe slurry, the high melting wax should be used in a finely dividedparticulate form, the particles having a maximum dimension not exceedingabout 351) microns. If any substantial quantity of the high melting waxcomponent comprises particles larger than 35;) microns, there is apossibility of forming wax spots in the final molded article anduniformity of resistance to oil penetration may not be achieved.

The low melting point wax component can easily be made up in the form ofan oil-in-water emulsion and is conveniently introduced into the slurryin this form. However, other techniques are equally suitable. Forexample, the wax component may be chilled (with liquid nitrogen or anyother suitable coolant), ground to the desired size and then introducedas a dispersion. As in the case of the higher melting wax component, thelower melting wax is preferably added to the stock at any time beforeformation. Addition to the beater stock has been found to be preferredin the practice of this invention.

In making emulsions of the lower melting Wax com ponents, it ispreferable to form an emulsion wherein the discontinuous phase (Wax)particles range from about 2 to 5 microns in size. Although largerparticles may be used, Wax dispersions having the larger particles areoften difficult to formulate.

The high melting and low melting point Wax componcnts may beconveniently introduced into the slurry as a single preparation. Thiscan be accomplished by preparing a dilute emulsion of the low meltingpoint wax component and then dispersing the high melting point Waxtherein. An alternative method of introducing the Wax components as asingle preparation is to be preblend them by melting and subsequentsolidification before introduction into the slurry. A combination ofthese methods may also, of course, be used.

The methods used in forming a molded cellulose article are well known tothose skilled in the art. US. Patents 2,017,617, 2,257,573 and 2,752,830are examples of ap paratus suitable for forming and drying the articlesof the present invention. The drying operation may also be accomplishedby the use of conventional tunnel drying, i.e., without the applicationof pressure.

Additional additives or treating agents may be added to the slurry toimpart other desirable characteristics to the finished molded pulparticle of the present invention. As an example, agents such as resinsor modified rosins may be added to decrease the penetration of aqueousliquids. When these are added, it is usually desirable to exhaust theresins onto the fibers by well known methods, e.g., to lower the pH ofthe slurry by adding a material such as paperrnarkers alum. The rosinsor modified rosins may be exhausted onto the fibers in the slurry beforeor after the wax components of this invention are added and they aregenerally present in concentrations up to about 2.5% by weight of themolded article.

It is also within the scope of this invention to add other treatingagents, such as finely divided particulate matter, to increase thesurface area and thereby improve wax retention, improve strength ormodify other physical properties of the finished molded article. Forexample, finely divided silicates may be introduced onto the fibers toincrease the surface area and to impart strength to the article. This isillustrated by Example VI hereinafter.

Such finely divided material should be of such a particle size that itssurface area is no greater, and preferably less than that of the fibersmaking up the cellulosic article. lfit is used to enhance resistance tooil penetration through increased surface area, it should be anoleophilic material, i.e., it should be wetted by oils.

Finally, it is, of course, possible to add pigments or dyestuffs to givethe article any desired color.

The invention may be further described by the following examples, whichare meant to be illustrative and not limiting.

Example 1 A furnish was prepared by charging a pulp beater with groundwood and unbleached sulfite pulp. The quantities used were such as toprovide, on a bone-dry basis, 92.5% ground wood and 7.5% by weightunbleached sulfite. The consistency of the furnish was made up toapproximately and it was beaten until a Canadian freeness ofapproximately 200 was achieved.

Neutral rosin size was added to the heater and thorough ly dispersed.The quantity of rosin present was equivalent to about 1% by weight ofthe fibers. After the rosin had been thoroughly dispersed, a solution ofpapermakers alum was added to the beater in an amount sufficient toreduce the pH of the furnish to between 4.1 and 4.5.

In separate operations, a dispersion of Acrawax C and an emulsion ofAristowax wax were made up. The Acrawax C had a melting point of 284 F.,a flash point of 545 F. and may be further characterized as a complexnitrogen derivative of higher fatty acids. The Aristowax had a meltingpoint of 160 to 165 F., and was a hydrocarbon paraffin wax.

In preparing the Acrawax dispersion, 1.6 grams of stearic acid wasmelted and added to 1.2 grams of monoethanolamine together with 2.2grams of water while the temperature was maintained between 70 and 75 C.After a period of about 5-10 minutes, 60 grams of water heated to 70 to75 C. was added. To the resulting mixture were then added 3 grams ofisopropanol and finally 32 grams of Acrawax ranging between 5 and 100microns in size. The dispersion was stirred until it had cooled toapproximately room temperature.

The emulsion of the lower melting point Aristowax was made by melting 22grams of Aristowax with 1.81 grams of stearic acid. The mixture was thenbrought to a temperature of about 205 to 210 F. To this were added 0.38gram of monoethanolamine and 1.2 grams of water. Finally an additional74.6 grams of water, heated to boiling, was added with rapid stirring toform an oil-in-water emulsion.

A quantity of the Acrawax C dispersion prepared as described above wasthen added to the beater in an amount suflicient to provide theequivalent of 1.5% by weight Acrawax C on the dry weight of the fibers.Following the addition of the higher melting point wax, a portion of theemulsion of the Aristowax was added to the furnish in the beater in anamount to provide 0.5% by weight of the fibers, resulting in theinclusion of 2% total wax by fiber weight. The furnish was then pumpedto a slurry pit and diluted to a consistency of approximately 1%. Moldedpulp plates were then prepared by the process described in US. Patent2,017,017, the temperatures of the drying dies grading from 350 to 500F.

In order to evaluate the ability of the resulting plates to resistpenetration of cold and hot oils, corn oil at 35 F. and at 240 F. waspoured into plates made in accordance with this example. It required twohours for the cold oil and 24 hours for the hot oil to penetrate throughthe plates to the bottom where the oil was visibly present. In actualuse these plates are suitable for resisting peneration of oil over atemperature range from 35 to 260 F.

In contrast to the performance exhibited by the plates prepared inaccordance with this example, plates prepared by standard procedurescontaining wax melting at 165 F. permitted the corn oil at 35 F. as wellas the corn oil at 160 F. to penetrate in two minutes.

Example I] A furnish was prepared as in Example I to the point where therosin of that example was added. In place of the rosin of Example I amalcic anhydride modified rosin (sold as Pexol by Hercules PowderCompany) was added to the beater to furnish sufiicient of this modifiedrosin to equal 2% of the dry weight of the fiber. Alum was then added toreduce the pH to between 4.1 and 4.5. An Acrawax C dispersion and anAristowax emulsion were then added as in Example I above and moldedplates were prepared from the furnish. The resulting plates gave oilresistance comparable to that described in Example I.

Example 111 Plates were prepared from a furnish identical with that ofExample I except that a small quantity of green MX (Calco Chemical Co.)was added as a dyestuif. This dyestuft is understood to be malachitegreen. The resulting plates were a pleasant green and possessed the sameability to resist penetration of oil as the plates of Example 1.

Example IV A furnish was prepared as in Example I except that asulficient quantity of the Acrawax C dispersion was added to provide12.0% by fiber weight and enough Aristowax emulsion was added to provide4.0% by fiber weight, giving a total wax content of 16.0% by weight ofdry fibers.

The rate of cold and hot corn oil peneration for the plates preparedfrom this furnish was greater than 24 hours for corn oil at 240 F. andgreater than two hours for corn oil at 72 F.

Example V A furnish like that described in Example I was prepared exceptthat pure ethylene bis-(stearamide) (melting point 284 F.) was used inplace of the Acrawax C and was added in an amount equivalent to 2.0% byweight of the fiber and a hydrocarbon paraffin wax melting betweenl50155 F. was substituted for the Aristowax of that example in an amountequivalent to 0.5% by weight of the fiber. When plates were preparedfrom this furnish, the results were distinguishable from those ofExample I in that the penetration of corn oil at an initial temperatureof 240 F. took more than 24 hours; while corn oil at 72 F. penetratedthe plate in somewhat less than two hours.

Example VI To the furnish prepared in accordance with Example I abovewas added quantity of sodium silicate solution equivalent to about 7.5%based on the dry weight of the fiber. When plates were molded from thisslurry, the strength of the plate was greatly increased, while the oilresistance showed improvement over the product of Example I in that itrequired more than 24 hours for corn oil at 240 F. and more than 2 hoursfor corn oil at 35 F. to penetrate.

Example VII Plates were prepared from a furnish as in Example I in whichthe low melting Aristowax was replaced with an equivalent weight ofmicrocrystalline wax having a melting point of F. The resulting platesexhibited approximately the same degree of resistance to penetration asthose prepared in Example I. Likewise when the Aristowax was substitutedwith an equivalent weight of paraffin having a melting point of 128 F.the resulting paper plates showed the same good resistance to thepenetration of corn oil.

However, when the Aristowax was replaced with an equivalent amount ofmicrocrystalline wax having a melting point between and F. (i.e., abovethe 165 F. specified), the resulting paper plates did not exhibitresistance to the penetration of the cold oil as in the case of ExampleI.

The process of this application is applicable to making moldedccllulosic articles from mechanical pulp as illustrated in Examples Ithrough VII and from chemical pulp as illustrated in the followingExample VIII. Moreover, the process is also, of course, applicable totreating so-called semichemical pulp.

Example VIII A furnish having a consistency of about was prepared usingall bleached hard wood kraft pulp and containing 2% by fiber weight ofPexol (a maleic anhydride modified resin). The furnish was transferredto a Dynapulper Where it was defibered. To the resulting dispersion wasadded a suflicient amount of alum to give a final pH of 4.1 to 4.5. Thefurnish was then run through a Jordan to give a Canadian standardfreeness of 300 to 325. To this was then added an Acrawax dispersionprepared as in Example I and an Aristowax emulsion prepared as in thatexample. The higher melting point wax component was added in an amountsufilcient to furnish 1.5% by fiber weight while the lower meltingcomponent was added in an amount sutficient to furnish 0.5% by weight ofthe fiber. The total amount of wax added was therefore equivalent to 2%of the fiber weight. The furnish was then diluted to a consistency of 1%and paper plates were prepared therefrom as in Example 1'. The resultingplates when exposed to corn oil at 35 F. and at 240 F. exhibited aperformance identical to that of the plates of Example I.

Example IX The Aristowax used in Example I as the low melting waxcomponent was treated to remove substantially all of the liquidhydrocarbon constituents contained therein. This was done by subjectingthe wax to successive treatments with methyl ethyl ketone. The wax wasdissolved in the solvent at a temperature equal to the boiling point ofthe solvent and the resulting solution was cooled to about 80 F. The waxwhich precipitated was filtered oif and was retreated by this methodthree more times. At the end of these four successive treatments theliquid hydrocarbon constituents had been reduced to 0.05% by weight ofthe wax. The treated Aristowax was then used in forming paper plates bythe process described in Example I and by using the same quantities asgiven in that example. The resulting plates were evaluated for theirability to resist penetration of cold corn oil at 35 F. after beingexposed to air for various periods of time. The results of these testsindicate that the plates made in this example can be stored for timesconsiderably in excess of one year while still maintaining theirexcellent resistance to the penetration of cold oils.

We claim: 1. A molded pulp product having substantially uniformlydistributed therethrough from about 1% to about 16% by weight of amixture of a first wax melting above about 212 F. and a second waxmelting below about 165 F., the amount of said first wax in said mixturebeing in excess of that amount which is compatible in the solid statewith said second wax when said waxes are melted together andsubsequently solidified and said mixture being present in said productas discrete wax phases of varying wax composition.

2. The molded pulp product of claim 1 including a surface areaincreasing amount of finely divided particulate matter substantiallyuniformly distributed therethrough.

3. A molded pulp product having substantially uniformly distributedthercthrough from about 1% to about 16% by weight of a mixture of afirst wax melting above about 212 F. and a second wax melting belowabout F., said first and second waxes exhibiting toward each other adegree of incompatibility in solid form and being present in saidmixture in a ratio such that after melting and subsequent solidificationthere are a plurality of discrete wax phases present, one phase beingsubstantially identical to said first wax, another phase beingsubstantially identical to said second wax and a third phase being ahomogeneous mixture of said Waxes having a melting point intermediatethe melting points of said first and second waxes.

4. A molded pulp product having substantially uniformly distributedthcrethrough from about 1% to about 16% by weight of a mixture of afirst wax melting above about 212 F. and a second wax melting belowabout 165 F., said first and said second waxes exhibiting toward eachother a degree of incompatibility in solid form and being present insaid mixture in a ratio such that after melting and subsequentsolidification there are a plurality of discrete wax phases present, onephase being subsequently identical to said first wax, another phasebeing substantially identical to said second wax and a plurality of waxblend phases which exhibit a melting point range from substantially thatof said second wax to substantialy that of said first wax.

5. The process comprising the steps of forming an aqueous slurry of pulpfibers, introducing into said slurry a mixture of a first wax having amelting point above 212 F. and a second wax having a melting point below165 F., depositing pulp fibers including said waxes in the form of astructural shape on a mold, and drying resulting structural shape at atemperature above the melting point of said first wax, said first andsaid second waxes exhibiting toward each other a degree ofincompatibility in solid form and being present in said mixture in aratio such that after melting and subsequent solidification there are aplurality of discrete wax phases present, one phase being substantiallyidentical to said first wax, another phase being substantially identicalto said second wax and a third phase being a homogeneous mixture of saidwaxes having a melting point intermediate the melting points of saidfirst and second waxes.

References Cited in the file of this patent UNITED STATES PATENTS1,897,694 Ellis Feb. 14, 1933 2,601,597 Daniel et al June 24, 1952FOREIGN PATENTS 612,635 Canada Jan. 17, 1961 OTHER REFERENCES Bennett:Commercial Waxes, Chemical Publishing Co. Inc., New York, pp. 173-174and 661 (1956).

1. A MOLDED PULP PRODUCT HAVING SUBSTANTIALLY UNIFORMLY DISTRIBUTEDTHERETHROUGH FROM ABOUT 1% TO ABOUT 16% BY WEIGHT OF A MIXTURE OF AFIRST WAX MELTING ABOVE ABOUT 212*F. AND A SECOND WAX MELTING BELOWABOUT 165*F., THE AMOUNT OF SAID FIRST WAX IN SAID MIXTURE BEING INEXCESS OF THAT AMOUNT WHICH IS COMPATIBLE IN THE SOLID STATE WITH SAIDSECOND WAX WHEN SAID WAXES ARE MELTED TOGETHER AND SUBSEQUNTLYSOLIDIFIED AND SAID MIXTURE BEING PRESENT IN SAID PRODUCT AS DISCRETEWAX PHASES OF VARYING WAX COMPOSITION.